1. Field of the Invention
The present invention relates to a switching circuit, and more particularly to an improvement of a switching circuit which is preferably used for switching an input/output of a high frequency signal.
2. Description of the Related Art
Recently, mobile communication systems such as a mobile telephone system, a pocket telephone, and so on, have been developed as a business on a large scale. On the other hand, shortage of communication circuits is becoming more and more serious in city areas, and various mobile communication systems are coming into practical use in various countries. Not only analog communication systems but also digital communication systems are often adopted as these communication systems, and, as for a communication frequency band, a submicrowave band is used which is on the higher frequency side than that in current mobile communication systems.
Semiconductor field-effect transistors (FETS) are often used in a signal processing portion of a portable terminal in such a communication system which transmits and receives signals of a submicrowave band. Particularly, in the case of a portable terminal in which portability is regarded as important, it is regarded as important to develop a monolithic microwave IC (hereinafter abbreviated to `MMIC`) using a GaAs-FET, by which the portable terminal can be made small in size, low in driving voltage and low in power consumption. Particularly, a high-frequency switching device for switching a high-frequency signal in a portable terminal is becoming a key device.
In most mobile communication systems which have come into use, digital systems are adopted as mentioned above. Of those digital systems, a TDMA (Time Division Multiple Access) system is often used. In this TDMA system, a communication band is divided by a predetermined time unit, and the divisional time sections are distributed to transmission, reception and another circuit. In the terminal side, therefore, in order to prevent transmission and reception from being conducted simultaneously, a switching circuit is often used for switching an antenna terminal between a transmission portion (Tx) and a reception portion (Rx).
An example of such a communication terminal device is shown in FIG. 1. When there is much loss of a transmission signal W1 or a reception signal W2 in this switching circuit SW, the quality of the signal deteriorates correspondingly. To avoid this deterioration, it is necessary to increase the signal power correspondingly to the loss. It is therefore preferable to make the loss in the switching circuit SW as small as possible.
In addition, there is a possibility that devices of the reception portion 3 are broken if a transmission signal W1 leaks by a large quantity to the reception side. In addition, if the isolation between the transmission portion 4 and the reception portion 3 is insufficient, signals may be distorted. It is therefore necessary to provide enough isolation between the transmission portion 3 and the reception portion 4. Since superior high-frequency characteristic and high speed switching are thus required of the switching circuit SW, GaAs-FETs are often used as switching devices.
When an FET is used as a switching device, a gate bias which is much higher than a pinch-off voltage of the FET is applied to the gate of the FET so as to make the impedance low between the drain and source to thereby control the FET to be into an ON state. On the contrary, a gate bias which is much lower than the pinch-off voltage of the FET is applied to the gate so as to make the impedance high between the drain and source to thereby control the FET to be into an OFF state.
In the case of a GaAs-FET available on the market recently, the FET can approximate a resistance component connected between the drain and source in an ON state while the FET can approximate a capacitance component connected between the drain and source in an OFF state. Then the resistance value and capacitance value of the FET can be regarded as several [.OMEGA./mm] and several hundred [fF/mm] per gate width (Wg) of the FET, respectively. For example, the resistance R.sub.on is 2 [.OMEGA./mm], and the capacitance C.sub.off is 300 [fF/mm].
When a submicrowave signal is switched by use of such an FET singly, enough isolation cannot be provided if loss is restrained sufficiently. On the other hand, if enough isolation is ensured, loss is increased. That is, in order to realize a sufficiently small insertion loss, it is necessary to increase the gate width of the FET to some extent so as to reduce the ON-resistance, but on the other hand the capacitance between the drain and source at the time of OFF is increased when the gate width is increased, so that there is a fear that the isolation deteriorates.
Therefore, when a microwave signal is switched, often an FET1 is disposed in a series position with respect to a signal path, and an FET2 is disposed in a shunt position between the signal path and the ground, as shown in FIG. 2A. For example, when a signal band is 2 [GHz], insertion loss of 1 [dB] or less and isolation of 20 [dB] or more can be easily ensured by a switching circuit constituted by a GaAs-FET disposed in such a series position and a GaAs-FET disposed in such a shunt position.
When an FET is disposed in a shunt position as shown in FIG. 2A, the shunt FET is connected to the ground, so that the potential in each of the drain region and the source region of the FET is 0 [V] in DC so long as the FET is not separated in view of DC by a capacitor or the like as shown in FIG. 2B. Generally speaking, in order to turn a GaAs-FET off sufficiently, it is necessary to apply negative gate bias to the drain region and the source region.
Therefore, a negative power supply is required to control the FET. However, if such a switching circuit is used as a terminal of a cellular communication or a portable telephone as mentioned above, the negative power supply needs a DC-DC converter, and so on, disadvantageously in cost, size and power consumption. Therefore, a device using no negative power supply is being performed upon a GaAs monolithic microwave IC (hereinafter abbreviated to `MMIC`) at present.
In the case of this IC, a capacitance is provided between a shunt FET and the ground in a chip, and an FET in a switching circuit is isolated from the ground in view of DC. However, in this case, the capacitance provided for DC cut is considerably large in size in a submicrowave region. It is therefore disadvantageous in producing an inexpensive IC.
In the case of a switching circuit for switching a signal among several places by means of FETs, such as an SPDT (Single Pole Dual Throw) switching circuit, there is a problem that insertion loss cannot be reduced to a certain value or less even if the size of isolation is ignored.
This will be described in an SPDT switching circuit in FIG. 3. For example, the signal path between RF1 and RF2 is in an ON state (that is, an FET1 is in an ON state and an FET2 is in an OFF state), a signal passing the signal path between RF1 and RF2 leaks through an OFF capacitance Cds of the FET2 which is in an OFF state, so that the insertion loss deteriorates.
Therefore, in the case where the gate width of the FET1 is made to be equal to that of the FET2, even if their gate widths are increased to reduce ON resistance of one FET which is in an ON state so as to reduce the signal loss of the FET which is in an ON state, the OFF capacitance Cds of the other FET which is in an OFF state increases to thereby increase the signal leaking from the FET in an OFF state, so that the insertion loss is not reduced beyond a certain value.
Therefore, in the switching circuit for switching a signal among a plurality of directions by use of FETs as shown in FIG. 3 (other than an SPST (Single Pole Single Throw) switching device which is an ON/OFF switch, there is a limit also in reducing insertion loss.